In recent years, various types of electronic devices are mounted with wireless communication functions. For example, in a transmitter for wireless communications, a power amplifier is being used to transmit radio waves in the air. Due to outputting signals having large power, the power amplifier is a block consuming large power in the transmitter.
Therefore, in a battery-driven mobile terminal such as a smart phone, a tablet (computer), and the like, it is preferable to reduce power consumption by enhancing power efficiency of a power amplifier. The power amplifier is roughly divided into a linear mode power amplifier and a switch mode power amplifier.
The linear rode power amplifier outputs an input high frequency signal (high frequency input signal) via linear amplification, and it is possible to allow the power amplifier to have a simple circuit configuration since not only phase information (a phase signal) but also amplitude information (an amplitude signal) are able to be simultaneously amplified.
The switch mode power amplifier is capable of amplifying only a phase signal since a transistor is switching-operated and therefore, to amplify an amplitude signal, a separate circuit is provided. For example, in an EER (Envelope Elimination and Restoration) system, a high frequency input signal is separated into a phase signal and an amplitude signal and the switch mode power amplifier is driven by the phase signal. The amplitude signal is amplified via power supply modulation.
The switch mode power amplifier ideally has higher power efficiency than the linear mode power amplifier. The reason is that ideally, during application of a voltage to the drain of a transistor (switch) in the switch mode power amplifier, no drain current flows and in reverse, no drain voltage is applied while drain current flows. This fact is based on the relationship: power consumption=drain voltage×drain current=0.
As describe above, the EER system utilizes a signal generation circuit for generating an amplitude signal and a phase signal from an input high frequency signal (or for separating the latter into the former). To generate an amplitude signal in the signal generation circuit, a rectification circuit employing a diode or a rectification circuit utilizing voltage-current characteristics (Vgs-Id characteristics) of a MOS transistor is being used.
In such a signal generation circuit, distortion components are generated due to the voltage-current characteristics of the diode or the MOS translator. In other words, when a signal generation circuit (amplitude information generation circuit) having large distortion is used for EER, a power amplifier output after combining an amplitude signal and a phase signal by the switch mode power amplifier is also distorted. Therefore, for example, in a wireless communication device, information communications at a low error level become difficult.
In this regard, various types of EER systems for driving a switch mode power amplifier and various types of circuits for generating an amplitude signal from a high frequency input signal has been proposed.
Non-Patent Document 1: Feipeng Wang, et al., “Design of wide-bandwidth envelope-tracking power amplifiers for OFDM applications,” IEEE Microwave Theory and Techniques Society, pp. 1244-1255, April 2005
Non-Patent Document 2: David Su, et al., “An IC for Linearizing RF Power Amplifiers Using Envelope Elimination and Restoration,” Communications and Optics Research laboratory, HPL-98-186, November, 1998